1. Field of the Invention
This invention relates to a method of temperature control in the catalyst regenerator of a fluid catalytic conversion process. More particularly, it relates to a method of removing heat from the bed of solid catalyst particles undergoing regeneration by combustion of carbonaceous deposits on the catalyst surface with an oxygen-containing gas in the regenerator of a fluid catalytic cracking unit.
2. Description of the Prior Art
On most fluid catalytic cracking units, it is necessary to remove heat at a controllable rate from the regenerator where spent catalyst is regenerated by burning off carbonaceous material with an oxygen-containing gas, such as air, in order to maintain equilibrium cracking conditions since the exothermic heat of regeneration imparted to the catalyst is transmitted to the fresh oil feed to the cracking reactor. It is also necessary to remove heat continuously to prevent undue regeneration temperature levels tending to sinter or deactivate the catalyst by surface area reduction. When heavy hydrocarbon feeds, such as atmospheric residua, vacuum residua, and heavy crude oils are catalytically cracked, a greater amount of carbon deposits on the catalyst particles than in the cracking of feeds such as gas oil. When the spent catalyst from the catalytic cracking of such heavy hydrocarbon feeds is regenerated by combustion with an oxygen-containing gas in a regeneration vessel, the heat removal problem is further aggravated since more heat is released in the regenerator than that which can be utilized in the process.
Various methods have been proposed to remove heat from a regenerator. The heat removal has been accomplished by withdrawing a portion of the catalyst from the bed of catalyst in the regenerator and circulating it through a tubular waste heat boiler so as to cool it before returning it to the catalyst bed. It has also been proposed to install steam coils in the regenerator as a means of heat recovery and temperature control (see, for example, U.S. Pat. No. 2,853,455; U.S. Pat. No. 2,926,143; U.S. Pat. No. 2,777,804; U.S. Pat. No. 3,886,060).
A disadvantage of using steam coils in a bed of catalyst undergoing regeneration is that the coils cannot be allowed to go to dryness since the temperature of the metal increases greatly from the wet to the dry zone. This causes severe thermal stresses in the steam coil and frequent metal failures. Therefore, the coil is usually made to operate either fully wet or fully dry.
When the coil is made to operate fully wet, the amount of heat that can be removed from the process cannot be varied to any great extent at normal pressure levels. If pressures were raised to critical or above, there could be considerable variation, but this pressure is too high to be considered feasible from an economic standpoint. Raising the pressure level from, for example, 150 to 650 pounds per square inch gauge (psig) would raise the water temperature in the coil from 366.degree. to 499.degree. F. This would vary steam production in a 1,250.degree. F. bed by only 15 percent, which is not adequate for control. When the coil is operated dry, it means that steam is merely being superheated in the coil and that an external boiler is required to generate the steam which is subsequently passed into the coil.
It has now been found that the above-stated disadvantages can be minimized by utilizing, in a regenerator bed, a system of steam coils having two different pressures. One steam coil is maintained internally wet, whereas the second steam coil is maintained internally dry.